In Patent Literature 1, a rotary support member is arranged in the rear part of the front surface plate of a cutter head so as to rotate about an axial center vertical to the front surface plate. Further, a mounting hole is formed in a position of the rotary support member facing an opening formed on the front surface plate. A replacement bit is slidably contained in the mounting hole. During the replacement of bits, after the replacement bit is retracted from the opening of the front surface plate and is contained in the mounting hole of the rotary support member, the rotary support member is turned 90 degrees, so that the opening is closed by the closing surface portion of the front surface of the rotary support member.